Process for producing a boron-containing silicon carbide powder

ABSTRACT

Silicon carbide particles are produced by reacting a gaseous silicon compound or granular silicon with a carbon compound at a high temperature. In the reaction, the amount of free carbon content in the resultant silicon carbide particles can be controlled by monitoring the amount of unsaturated hydrocarbon such as acetylene, as a by-product. Moreover, silicon carbide particles can contain boron dispersed uniformly in the particles by a two step process comprising first reacting a silicon source and a boron source without a carbon source in a first reaction zone, to form boron-containing silicon particles, and second, reacting the resultant particles with a carbon source in a second reaction zone. Further, the above-mentioned monitoring of an unsaturated hydrocarbon by-product allows the obtaining of silicon carbide particles containing no free carbon, and the silicon carbide particles containing boron in the particles but no free carbon may be sintered without the addition of free carbon, to give a dense sinter.

This is a division of application Ser. No. 847,940 filed Apr. 3, 1986now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for producing siliconecarbide particles suitable for making a dense silicon carbide sinter.More specifically, it relates to a process for producing silicon carbideparticles in which a free carbn content in the silicon carbide particlesis controlled by monitoring a by-product such as acetylene. The presentinvention also relates to a process for producing a silicon carbidesinter from silicon carbide particles obtained by the above process.

2. Description of the Related Art

Silicon carbide has attracted attention as an excellent high temperaturestrength material suitable for use in gas turbines, etc. It is howeverdifficult to sinter a silicon carbide body to a density close to thetheoretical density of 3.21 g/cm₃. Processes for obtaining a densesilicon carbide sinter have been proposed, for example, U.S. Pat.application Ser. No. 409073 filed on Oct. 24, 1973, in which aboron-containing compound in an amount corresponding to 0.1 to 3.0 wt %of boron and a carbon source corresponding to 0.1 to 1.0 wt % ofelemental carbon as densifying agents are dispersed uniformly withsubmicron β-type silicon carbide particles, the resultant uniformdispersion is formed into a shape, and the shape is fired to obtain adense silicon carbide sinter. To produce submicron β-type siliconcarbide particles, a gaseous trichloromethylsilane and hydrogen, or asuitable gaseous hydrocarbon such as silicon trichloride or toluene andhydrogen, are introduced into an argon plasma generated between twoconcentric electrodes to produce silicon carbide crystallites having asize of 0.1 to 0.3 μm. It is also disclosed that free carbon may becontained in silicon carbide particles by using the carbon source in anamount slightly larger than the stoichiometric amount necessary forproducing silicon carbide.

It is known that, to obtain an excellent silicon carbide sinter, siliconcarbide particles containing boron and free carbon, desirably uniformlydispersed in the particles, as densifying agents, are preferable to amixture of silicon carbide particles and densifying agents such as aboron source and a carbon source. The former allows a uniform structureof a sinter and an improvement of the mechanical properties of a sinter.Thus, the above U.S. patent application Ser. No. 409,073 discloses thatfree carbon may be contained in silicon carbide particles in a processfor synthesizing the silicon carbide particles.

The amount of densifying agent is critical to the characteristics of asinter. If the amount of densifying agent is not appropriate, a goodquality sinter cannot be obtained. Therefore, when the free carbonproduced in silicon carbide particles during the synthesis of siliconcarbide particles is used as a densifying agent, control of the amountof the free carbon produced in the silicon carbide particles is veryimportant.

It is, however, impossible to concurrently determine a quantitativeamount of free carbon produced during the synthesis of silicon carbideparticles. Thus, the amount of free carbon in silicon carbide particlesis analyzed after a certain amount of silicon carbide particles havebeen synthesized to determine whether or not the amount of free carbonproduced during the synthesis is appropriate. Further, to control theamount of free carbon produced in silicon carbide particles inaccordance with the results of the above analysis, it is necessary topreliminarily examine the relationships between the reaction conditionsand the amount of free carbon, by synthesizing silicon carbide particlesunder various reaction conditions. This necessitates a large number ofexperiments. Here, the reaction conditions include temperature,pressure, feed of raw materials, shape of a reaction chamber, etc.

Even if the reaction is conducted in specified conditions, the amount offree carbon in the silicon carbide particles may vary with the amount oftime lapsed, because silicon carbide particles, etc., are deposited onthe inside wall of a reaction chamber and, as a result, the residencetime of a raw material in the reaction chamber and other factors arevaried. This makes it difficult to produce free carbon in a desiredamount.

U.S. patent application Ser. No. 471,303 filed on May 20, 1974 disclosesa process for producing particles comprising a uniform dispersion ofβ-type silicon carbide, boron and free carbon, in which a gaseousmixture essentially consisting of a silicon halide, a boron halide, anda hydrocarbon is introduced to a plasma jet reaction zone. Thisapplication suggests the effectiveness of a concurrent addition of boronhalide during the synthesis of silicon carbide particles.

If a silicon compound, a carbon compound, and a boron compound areconcurrently introduced into a single high temperature reaction zone, asin the U.S. patent application Ser. No. 471,303, mainly, silicon carbideis grown onto silicon carbide seeds, to give submicron silicon carbidecrystals, and boron carbide or boron is grown onto boron carbide orboron seeds to give submicron boron carbide or boron crystals,respectively. Almost all of the boron is not doped in silicon carbideparticles. In other words, high temperature stable extremely small seedcrystals of silicon carbide, boron carbide and boron are first producedrespectively, and then silicon carbide, boron carbide and boron aregrown around these seed crystals, respectively, maintaining the samesrystal structure, resulting in the formation of submicron siliconcarbide, boron carbide and boron particles, respectively. In such acase, boron is not always contained in silicon carbide particles buttends to form boron carbide particles or boron particles, although asilicon compound, a carbon compound and a boron compound are added in asingle reaction zone. Thus, almost all of the boron is not disperseduniformly in the silicon carbide particles and it is difficult to obtansilicon carbide particles with boron uniformly dispersed therein.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a process for obtaininguniform silicon carbide particles containing a desired amount of freecarbon by monitoring the amount of produced free carbon in siliconcarbide particles indirectly but in real time during the synthesisthereof and controlling the amount of free carbon accordingly.

Another object of the present invention is to provide a process forproducing silicon carbide particles containing boron which correspondsto almost all of a boron source fed and is dispersed uniformly in theparticles.

A further object of the present invention is to provide a process forproducing a silicon carbide sinter, which process produces a densesinter having uniform and improved characteristics.

A still further object of the present invention is to provide a processfor producing a silicon carbide sinter, which process is simple andprovides a sinter having improved characterstics.

The above and other objects of the present invention are attained by aprocess for producing silicon carbide particles from a reaction betweena gaseous silicon compound or particulate silicon and a gaseous carboncompound at a high temperature, in which an amount of an unsaturatedhydrocarbon produced as a by-product during the reaction is monitoredand the conditions of the reaction are controlled in accordance with themonitored amount of the unsaturated hydrocarbon by-product, to regulatethe amount of the unsaturated hydrocarbon by-product to a certain range,whereby the content of free carbon in the silicon carbide particles iscontrolled.

If an amount of a carbon compound slightly in excess of thestoichiometric amount is used in the synthesis of silicon carbideparticles from the silicon compound and the carbon compound, the carboncompound in an amount of the stoichiometric amount is reacted with thesilicon compound, and the silicon carbide is produced. On the otherhand, a portion of the carbon compound in excess of the stoichiometricamount produces free carbon and the rest of the carbon compound isemitted from the reaction system as a gas.

A gas emitted from the reaction system includes the carbon compound ofthe starting material and another carbon compound resulted from thestarting carbon compound, for example, an unsaturated hydrocarbon suchas acetylene.

The present inventors found that there is a strong relationship betweenthe amount of unsaturated hydrocarbon as a by-product and the amount offree carbon as a product. If the amount of an unsaturated hydrocarbonby-product is larger, the amount of free carbon product is also larger.The relationship between these appears repeatedly.

By an equipment capable of analyzing a gas in an extremely short timeperiod, such as a mass spectrograph or a gas chromatograph, the amountof an unsaturated hydrocarbon by-product can be measured in real time,which allows the detection of the amount of the free carbon product inreal time. As the amount of the free carbon product can be detectedduring the synthesis of silicon carbide in real time, it is possible tocontrol the amount of the free carbon product to an appropriate value orrange at all times. That is, once the relationship between the amount ofan unsaturated hydrocarbon by-product and the free carbon product isdetermined by experiments, controlling the reaction to make the amountof the unsaturated hydrocarbon by-product to a certain value results inproducing a certain desired amount of free carbon. In order to controlthe reaction, the feeding rate of a carbon compound of the startingmaterial is ordinarily varied, but the variation of other factors suchas temperature and pressure also may be used.

Regarding the reasons why such a strong relationship is present betweenthe amount of free carbon product and the amount of an unsaturatedhydrocarbon by-product, we considered the following: That is, becausethe reaction conditions of producing, an unsaturated hydrocarbon from acarbon compound of the starting material have strong similarities tothose of producing the free carbon, or because an unsaturatedhydrocarbon is an intermediate reaction product of a free carbonproducing reaction. The mechanism of the production of free carbon,however, has not been made clear at present.

In a process according to the present invention, silicon carbideparticles containing substantially no free carbon can be produced.

In an embodiment of this process, the particulate siliconmentioned-above may be prepared by introducing a silicon compound to afirst reaction zone at a temperature higher than a melting point ofsilicon to form fused spherical silicon particles. The resultant fusedspherical silicon particles are then reacted with a carbon compound in asecond reaction zone at a temperature of less than a boiling point ofsilicon to produce silicon carbide particles. This two step processallows the production of desirably spherical silicon carbide particles.

In the above two step process, silicon carbide particles containing asmall amount of boron uniformly distributed in the particles can beproduced. Such boron-containing silicon carbide particles can beprepared by introducing silicon or a silicon compound containing nocarbon together with boron or a boron compound containing no carbon intoa first reaction zone at a temperature higher than the melting point ofsilicon to form fused boron-containing silicon particles. The resultantfused boron-containing silicon particles are then reacted with thecarbon compound in a second reaction zone at a temperature of less thanthe boiling point of silicon to produce silicon carbide particlescontaining a small amount of boron.

Preferably, the silicon or silicon compound and the boron or boroncompound are preliminarily mixed together before being introduced to thefirst reaction zone.

The silicon carbide particles preferably contain boron, as a densifyingagent for sintering silicon carbide particles, in a range of from 0.1 to5.0% by weight.

Thus obtained boron-containing silicon carbide particles can be sinteredwithout the addition of any carbon source even if the silicon carbideparticles do not contain free carbon, as explained later in detail.Boron-containing silicon carbide particles containing substantially nofree carbon can be produced in a process described before, i.e., in atwo step process, by monitoring an unsaturated hydrocarbon by-product.

Thus, according to the present invention, there is provided a processfor producing a silicon carbide sinter, comprising the steps ofpreparing silicon carbide particles containing 5% or less by weight ofboron and substantially no free carbon, forming a shape of theboron-containing silicon carbide particles without the addition of acarbon source, and firing the shape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional side view of an apparatus for producing siliconcarbide particles in a reaction zone according to the present invention;

FIG. 2 is a sectional side view of an apparatus of producing siliconcarbide particles in two reaction zones according to the presentinvention; and

FIG. 3 is a graph of the amount of free carbon content in siliconcarbide particles in relation to the amount of acetylene by-product inExample 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The strong relationship between the amount of free carbon product andthe amount of an unsaturated hydrocarbon by-product appears in a reactinof a gaseous silicon compound or particulate silicon with a gaseouscarbon compound to produce silicon carbide particles at a hightemperature. The silicon compound used includes, for example, silaneSiH₄, monochlorosilane SiH₃ Cl, dichlorosilane SiH₂ Cl₂, trichlorosilaneSiHCl₃, silicon tetrachloride SiCl₄ and disilane Si₂ H₆. Silane ispreferred.

Fine particulate silicon may be used in place of a gaseous siliconcompound. Particulate silicon having an average particle size of 100 to0.1 μm is preferred.

The carbon compound used includes, for example, an aliphatic hydrocarbonsuch as methane, ethane, propane, butane, ethylene, propylene andbuthylene, an cyclic hydrocarbon such as benzene, toluene andcyclohexane.

The unsaturated hydrocarbon used for monitoring includes, for example,acetylene. We found that an aliphatic hydrocarbon having 4 or lesscarbon atoms gives an especially strong relationship between the amountof free carbon product and the amount of acetylene by-product, and suchan aliphatic hydrocarbon is preferably used.

The unsaturated hydrocarbon used for monitoring may be a combination oftwo or more unsaturated hydrocarbon or even a combination of one or moreunsaturated hydrocarbon and one or more other unsaturated hydrocarbon,etc. Further, the unsaturated hydrocarbon may be a substitutedunsaturated hydrocarbon when the starting carbon compound is asubstituted hydrocarbon.

In a preferred embodiment of the present invention, silicon carbideparticles are produced by two steps. That is, a silicon compound isintroduced into a first reaction zone at a temperature higher than themelting point of silicon, 1685K, to form fused sperical siliconparticles, and then the resultant fused spherical silicon particles arereacted with a carbon compound in a second reaction zone at atemperature below the boiling point of silicon, 3492K, to form siliconcarbide particles. This two step process gives uniform spherical siliconcarbide particles suitable for obtaining a uniform and improved siliconcarbide sinter (see Japanese Kokai Publication No. 60-77114).

The present invention can be applied even if the particulate silicon asthe starting material contains a small amount of an additive. Forexample, the present invention can be applied where the siliconparticles contain a small amount of boron. In such a case, siliconcarbide particles can be produced by introducing silicon or a siliconcompound containing no carbon and boron or a boron compound containingno carbon into a first reaction zone at a temperature higher than themelting point of silicon, 1685K, to produce fused boron-containingsilicon carbide particles, and the resultant fused boron-containingsilicon carbide particles are then reacted with a carbon compound at atemperature below the boiling point of silicon, 2608K, to producesilicon carbide particles containing a small amount of boron.

In such a case, the mechanism of a reaction of silicon particlescontaining boron with a carbon compound to produce silicon carbideparticles containing boron is the same or almost the same as themechanism of a reaction of silicon particles with a carbon compound toproduce silicon carbide particles containing no boron. As a result, thepresent invention can be applied to both the former and the latterreactions. First, a relationship between the amount of free carbonproduct and the amount of an unsaturated hydrocarbon by-product in theconsidered reaction is preliminarily examined. This can be done by aconsiderably smaller number of experiments than the number ofexperiments needed to examine the amount of free carbon product undervarious reaction conditions. Next, during the synthesis, the reaction iscontrolled to make the amount of the unsaturated hydrocarbon by-productcorrespond to a desired free carbon product. If the amount of theunsaturated hydrocarbon by-product is smaller than a certain value, thefeeding rate of the carbon product is increased. If the amount of theunsaturated hydrocarbon is larger than the certain value, the feedingrate of the carbon compound is decreased. Thus, silicon carbideparticles containing a desired amount of free carbon and uniformlydispersed boron can be produced over a long period of time.

In the above synthesis, the silicon or a silicon compound used may bethe same as used in the synthesis of silicon carbide particlescontaining no boron.

The boron compound used may be any one that deposits boron in the firstreaction zone and includes, for example, diborane B₂ H₆, tetraboron B₄H₁₀ and boron trichloride BCl₃. A boron compound that relatively easilydeposits boron at a high temperature, such as diborane, is preferred.Fine boron particles may be used in place of a boron compound.

The ratio of boron to silicon may be selected according to theapplications of the resultant silicon carbide particles. To obtain agood silicon carbide sinter, the amount of boron may be 5% or less byweight, preferably 4% or less by weight, based on the amount of silicon.

The silicon or silicon compound and the boron or boron compound arepreferably introduced to the same portion of the first reaction zone.More preferably, they are preliminarily mixed in a given ratio beforebeing introduced to the first reactin zone. This allows a more thoroughmix of the silicon or silicon compound with the boron or boron compoundand allows a stable introduction of the boron or boron compound in awell defined ratio to the silicon or silicon compound in the firstreaction zone, due to the preliminary mixing. This is also preferredsince, if silane and diborane are used, poisoneous diborane will beburnt by ignition of the silane if a leak occurs.

The silicon compound and the boron compound thus introduced to the firstreaction zone are quickly decomposed and form silicon and boron. Thesilicon is just melted and forms fused silicon particles since thetemperature in the first reaction zone is above the melting point ofsilicon. Almost all the boron is ingested in the fused siliconparticles, since fused silicon has a tendency to ingest boron, and fusedsilicon particles containing uniformly dispersed boron are obtained. thetemperature in the first reaction zone is preferably below the boilingpoint of the silicon, in order to form silicon particles.

The fused silicon particles containing boron are then reacted with acarbon compound in a second reaction zone to form silicon carbideparticles containing uniformly dispersed boron. The second reaction zonepreferably has a temperature at which the fused silicon particles areliquid. The carbon compound used may be the same as that used in thesynthesis of silicon carbide particles, i.e., containing no boron. Thecarbon compound should not flow back into the first reaction zone since,if the carbon compound flows back into the first reaction zone, thecarbon compound will come into contact with the silicon compound and theboron compound and, as a result, boron carbide particles and boronparticles are easily produced in addition to silicon carbide particles,as described in relation to the related art. In the present invention,however, almost all of the boron is uniformly dispersed in the siliconcarbide particles, since boron and fused silicon, easily ingests boron,are formed together in the first reaction zone, wherein fused siliconparticles containing extremely uniformly dispersed boron are producedand are then carbonized in the second reaction zone by the carboncompound to obtain silicon carbide particles.

The present invention enables the synthesizing of silicon carbideparticles containing no free carbon. In this case, the correspondingamount of an unsaturated hydrocarbon by-product is considerably small.

Further, the present inventors found that a dense silicon carbide sintercan be produced from silicon carbide particles without the addition of adensifying agent such as free carbon, if the silicon carbide particlescontain boron. Thus, the present invention provides a process forproducing a silicon carbide sinter, comprising the steps of preparingsilicon carbide particles containing 5% or less by weight of boron andsubstantially no free carbon by a process described before, forming ashape of the boron-containing silicon particles without the addition ofa densifying agent such as free carbon, and firing the shape.

As described before, in the prior art, silicon carbide particlescontaining boron uniformly dispersed in the particles have not beenobtained in high temperature gaseous synthesis. The silicon carbideparticles produced by a process in the prior art do not contain boron inthe particles in practice, or only a portion thereof can contain boronin the particles. In contrast, according to the present invention,silicon carbide particles containing boron uniformly dispersedthroughout the particles are obtained. This feature of the inventionenables a dense silicon carbide sinter to be obtained without theaddition of a densifying agent such as free carbon.

Here, the sintering mechanism of silicon carbide with densifying agentsof boron and free carbon is considered. A silicon carbide sinter ispolycrystalline. This means that fine silicon carbide crystallites arein contact with each other on their different crystal planes. Thiscontact on different crystal planes means chemical bonding between atomsof the different crystal planes, but this is impossible with siliconcarbide in its original nature, since silicon carbide is a covalentbonding and the directions of the bonding hands of silicon and carbonare limited, and angle range within which they can move freely is 6degrees or less. As the directions of the bonding hands of silicon andcarbon at different crystal planes are completely different, and thebonding hands can move only within 6 degrees or less, it is almostimpossible for the bonding hands of the different crystal planes to bondtogether. Thus, chemical bonds between different crystal planes ofsilicon carbide crystallites are extremely difficult to form, and it istherefore practically impossible to produce a sinter of pure siliconcarbide.

If the bonding hands are made to be able to move in a range of more than6 degrees, and different crystal planes are made to be chemically boundto each other, the above obstacles are overcome. It is believed thatthis is the role played by boron. That is, boron is present near grainboundaries in a silicon carbide sinter and makes the directions ofbonding hands of silicon and carbon to be free in certain wider anglerange, and thus makes it possible to chemically bond different crystalplanes to each other. To achieve this, boron should be contained in thesilicon carbide crystal grains. It is believed that this is assisted byfree carbon. It is generally said that the role of free carbon issupposedly to remove the surface oxygen of silicon carbide particles,and as a result, boron is easily ingested by silicon carbide particles.

In short, it is considered that free carbon plays a role of impregnatingsilicon carbide with boron, and boron makes chemical bonding betweendifferent crystal planes of silicon carbide possible. If this is so, itcan be understood that, according to the present invention, siliconcarbide is sintered until a sinter becomes dense even in the absence offree carbon, if silicon carbide particles containing boron in theparticles are used.

Oxygen does not always affect a densification of silicon carbide, evenif surface oxygen, etc., is present, if boron can be ingested by thesilicon carbide particles or if boron is preliminarily contained in thesilicon carbide particles. In general, the high temperaturecharacteristics of a silicon carbide sinter are deteriorated by thepresence of oxygen in the sinter and therefore, preferably the amount ofoxygen in the silicon carbide particles is reduced.

Preferably, the free carbon should be omitted if free carbon is notnecessary to make a dense sinter of silicon carbide. Further, sinteringsilicon carbide without free carbon is a superior process to thesintering of silicon carbide with free carbon, since it is verydifficult to ensure that the amount of added free carbon is suitable, inmicroscale. Free carbon is consumed by surface oxygen, etc., but it isdoubtful that free carbon in an amount necessary and sufficient forremoving surface oxygen, etc., can be added correctly in microscale. Inmicroscale, it is rather considered that free carbon is present inexcess in some locations and is insufficient in other locations. Suchexcess free carbon acts in a manner extremely similar to permanent micropores and, therefore, is extremely detrimental. Thus, sintering siliconcarbide without free carbon, as in the present invention, not onlyachieves a simple process for obtaining a dense silicon carbide sinterbut also provides preferred characteristics of a sinter.

As described above, it is preferred that free carbon be omitted if freecarbon is not necessary for obtaining a dense sinter. In the prior art,it is however considerably difficult to control a process forsynthesizing silicon carbide in a high temperature gaseous phase in sucha manner that free carbon is completely absent. However, a process forsynthesizing silicon carbide according to the present invention allowsthe process to be controlled in such a manner that the produced siliconcarbide particles contain no free carbon. By the infraredabsorptiometric method after combustion and hydrogen hot extractiontechnique, no free carbon was detected in silicon carbide particlesobtained by a process as mentioned above according to the presentinvention.

Silicon carbide particles containing boron uniformly dispersed in theparticles but containing substantially no free carbon, can be obtainedby a two step process comprising first forming silicon particlescontaining boron but no carbon compound in a first reaction zone, andthen reacting the particles with a carbon compound to form a siliconcarbide particles containing boron but substantially no free carbon in asecond reaction zone. The first step can be attained by using a siliconsource and a boron source which do not contain a carbon source. Thesecond step is controlled by monitoring an unsaturated hydrocarbonby-product so that the produced silicon carbide particles containsubstantially no free carbon. The content of boron in silicon carbideparticles is preferably 5.0% by weight or less based on the siliconcarbide particles. More than 5.0% by weight of boron will disturb thesintering of silicon carbide.

The thus obtained silicon carbide particles containing boron dispersedthroughout the particles, but containing substantially no free carbon,can be formed into a shape and fired without the addition of adensifying agent such as free carbon, to give a dense silicon carbidesinter. Shaping can be carried out in any conventional manner. Firingcan be achieved, for example, by heating the shaped or compacted siliconcarbide particles in an atmosphere inert to silicon carbide at atemperature of 1900° C. to 2200° C. The pressure of the atmosphere maybe either above atmospheric pressure or at atmospheric or a reducedpressure. Further, the firing may be conducted by hot pressing. Since asinter may become denser under pressure, this process is preferable. Thedensity of a silicon carbide sinter should be 90% or more of thetheoretical density for practical applications. The present inventionprovides a silicon carbide sinter having a density of from 98% to morethan 99%.

FIGS. 1 and 2 illustrate examples of an apparatus for producing siliconcarbide particles according to the present invention. In FIG. 1, anelectrical discharge is generated between a cathode 1 and an anode 2. Agas is introduced from a gas inlet pipe 3 and excited to form plasma,giving a high temperature higher than the melting point of silicon in areaction zone 4. Into the thus-heated reaction zone 4, a siliconcompound or silicon is introduced from an inlet pipe 5 and a carboncompound from an inlet pipe 6. The silicon compound or silicon and thecarbon compound react with each other to form silicon carbide particles,which are recovered from an outlet 7. An exhaust gas emitted from thereaction zone 4 through the outlet 7 is analyzed to detect the presenceof an unsaturated hydrocarbon such as acetylene. The data for thedetection of an unsaturated hydrocarbon is compared with a predeterminedspecified value, which corresponds to a desired free carbon content inthe silicon carbide particles. According to the results of thiscomparison, the amount of carbon compound introduced from the inlet pipe6, for example, is modified if necessary, to give the specified value ofthe amount of the unsaturated hydrocarbon or the desired free carboncontent.

The apparatus shown in FIG. 2 is similar to the apparatus of FIG. 1except that the apparatus in FIG. 2 comprises two reaction zones 14 and17. A silicon compound or silicon and, if desired, a boron compound orboron are introduced to a first reaction zone 14 from inlet pipes 15 and16 to form fused silicon carbide particles, containing boron, ifintroduced. A carbon compound is introduced from an inlet pipe 18 to asecond reaction zone 17 to react with the fused silicon particles, andsilicon carbide particles containing, if desired, free carbon, areproduced therein. The resultant silicon carbide particles are recoveredthrough an outlet 19. A gas emitted from the second reaction zone 17through the outlet 19 is also analyzed to control the reactionconditions in the second reaction zone, as described with reference toFIG. 1. In FIG. 2, reference numeral 11 denotes a cathode, 12 an anode,and 13 a gas inlet pipe.

The apparatus for producing silicon carbide particles according to thepresent invention is not limited to those shown in FIGS. 1 and 2. Forexample, the method for heating a reaction zone or a first reaction zonemay be any of direct current plasma heating, high frequency plasmaheating, electric resistance heating, induction heating, micro waveheating, infrared heating, laser heating, etc. The second reaction zonemay be heated by any of the above methods, or heating may be eliminated,or sometimes it may be necessary to cool the second reaction zone. Thedetection of unsaturated hydrocarbon may be conducted by any methodwhich enables the unsaturated hydrocarbon to be analyzed in real time,or in a relatively short time period, for example, from several tens ofseconds to several minutes.

EXAMPLE 1

The apparatus used was the same as that shown in FIG. 1. Argon gas wasintroduced from a gas inlet pipe 3 at a rate of 20 l/min and anelectrical discharge was generated between a cathode 1 and an anode 2under the conditions of 30 V and 700 A to produce an argon plasma to areaction zone 4. Silane gas was introduced from an inlet pipe 5 at arate of 1 l/min and methane gas was introduced from an inlet pipe 6 at arate of 1 to 13 l/min. The diameter of the reaction zone 4 was 40 mm andthe temperature in the reaction zone was about 2000° C. Thus, siliconcarbide particles were produced in the reaction zone 4.

The resultant silicon carbide particles were recovered from an outlet 7,and an exhaust gas emitted from the reaction zone 4 through the outlet 7was analyzed by a quadrupole mass spectrometer and the amount ofacetylene produced was measured. The amount of acetylene produced wascalculated by comparing the peak intensity of acetylene obtained by aquadrupole mass spectrometry with that of pure methane, with the amountof pure methane set at 100. The amounts of the acetylene and the puremethane were normalized by the amount of argon introduced into theapparatus.

The conditions of synthesizing silicon carbide particles were selectedso that the amount of acetylene produced was from 0.25 to 6.00, with apitch of 0.25, as the above-mentioned peak intensity; the total numberbeing 24. The time period for the synthesis were about 3 hours,respectively. In each case, the amount of methane introduced wascontrolled so that a constant amount of acetylene was produced and thefree carbon content of the resultant silicon carbide particles wasalways constant.

X-ray diffraction revealed that the resultant particles were β-typesilicon carbide. The free carbon content of the silicon carbideparticles was analyzed by infrared absorptiometric after combustion andhydrogen hot extraction technique. Further, chemical analysis, etc., waseffected on the silicon carbide particles. As a result, it was foundthat only silicon carbide and free carbon were detected. The analyseswere conducted on the particles obtained during the first, intermediate,and final stages, respectively, and the free carbon contents of theparticles were the same.

The resultant relationship between the amount of free carbon product andthe amount of acetylene by-product is shown in FIG. 3. From FIG. 3, itcan be seen that the relationship between the two is almost constant.

EXAMPLE 2

The apparatus used was the same as that shown in FIG. 2. Argon gas wasintroduced from a gas inlet pipe 13 at a rate of 20 l/min and anelectrical discharge was generated between a cathode 1 and an anode 2under conditions of 30 V and 700 A to produce a plasma. Silane wasintroduced to the first reaction zone 14 from an inlet pipe 15 at a rateof 1 l/min and disborane was introduced therein from an inlet pipe 16 ata rate of 0.015 l/min, with argon gas as the carrier gas. The diameterof the first reaction zone 14 was 70 mm and the temperature was about2000° C. Fused silicon particles containing boron uniformly therein wereproduced in the first reaction zone 14.

The fused silicon particles were fed into a second reaction zone 17 andmethane was introduced from an inlet pipe 18 to the second reaction zoneat a rate of 1 to 1.3 l/min. The fused silicon particles were carbonizedin the second reaction zone at a temperature of 1700 to 1800° C., toproduce silicon carbide particles containing boron uniformly in theparticles.

The resultant silicon carbide particles were recovered from an outlet17. The exhaust gas emitted from the outlet 17 was analyzed by aquadrupole mass spectrometer.

The synthesis conditions and the methods of analyzing the free carboncontent were the same as in Example 1.

The resultant relationship between the amount of free carbon product andthe amount of acetylene by-product was the same as in Example 1.

X-ray diffraction revealed that the resultant particles were β-typesilicon carbide. By chemical analysis, etc., about 0.8% by weight ofboron was detected in addition to silicon carbide and free carbon. X-rayphotoelectron spectroscopy revealed the boron was elemental boron.

EXAMPLE 3

The synthesis of silicon carbide particles in this Example is similar tothat of Example 2. However, the diameter of the first reaction zone was60 mm and the temperature in the first reaction zone was about 2500° C.Methane was introduced to the second reaction zone at a temperature ofabout 2000° C. and a rate of about 1.1 l/min. The rate of methaneintroduction was controlled by monitoring the amount of acetyleneby-product so that the resultant silicon carbide particles contained0.6% by weight of free carbon.

The resultant particles extracted from the outlet had a grain size in arange of 0.1 to 0.7 μm and an average specific surface area of 9.6 m²/g, from observation by a transmission type electron microscope. X-raydiffraction showed that the particles were β-type silicon carbide. Bychemical analysis, etc., 0.8% by weight of boron and 0.6% by weight offree carbon were detected. Observation X-ray photoelectron spectroscopyshowed that the boron was elemental boron.

The resultant silicon carbide particles were then subjected tosintering. 100 g of the silicon carbide particles were added and mixedwith 5 g of oleic acid solved in acetone. The mixture was dried toevaporate acetone only. The resultsatnt particles were subject touniaxial forming in a metal die having a size of 5 cm at a pressure of90 kgf/cm². The resultant shape was dried at 250° C. to evaporate oleicacid. The shape then had a density of 2.05 g/cm² (by the Archimedeanmethod), corresponding to 64% of the theoretical density. The shape wassintered in a 1 atom argon atmosphere at 2160° C. for 2 hours. Thetemperature raising time from room temperature to 2160° C. was about 2hours and 30 minutes, and the sinter was allowed to cool to the roomtemperature after the above sintering at 2160° C.

The resultant sinter had a density of 3.16 g/cm³, which corresponds to98.4% of the theoretical density. The flexural strengths measuredaccording to the method of JIS R 1601 were 68 kgf/mm² at roomtemperature and 77 kgf/mm² at 1500° C.

For comparison, commercial β-type silicon carbide particles were addedwith boron and carbon black and sintered under the same conditions andprocedures as the above. As a result, the silicon carbide sinter hadflexural strengths of 55 kgf/mm² at room temperature and 66 kgf/mm² at1500° C. As can be seen, superior results were obtained by using siliconcarbide particles produced by a process according to the presentinvention. It is believed that this is because the particles obtained bythe present invention contain boron as a densifying agent extremelyuniformly in all particles.

EXAMPLE 4

The synthesis of silicon carbide particles in Example 3 was repeatedexcept that silane and diborane were preliminarily mixed in a ratio of1:0.015, followed by introducing the mixture through the inlet pipesinto the first reaction zone. The characteristics of the resultantsilicon carbide particles containing boron in the particles were similarto those in Example 3.

Further, the sintering in Example 3 was repeated for the resultantsilicon carbide particles. The resultant silicon carbide sinter had adensity of 3.17 g/cm³ (by the Archimedean method), which corresponds to98.8% of the theoretical density. The flexural strength of the sinterwere 71 kgf/mm² at room temperature and 81 kgf/mm² at 1500° C.

EXAMPLE 5

The synthesis of silicon carbide particles in Example 2 was repeatedexcept that methane was introduced to the second reaction zone at a rateof about 1 l/min and the rate of methane introduction was controlled sothat no free carbon was produced, in accordance with the amount of theacetylene by-product which was monitored. Further, the concentration ofoxygen in the first and second reaction zones was kept below 0.1 ppm.

X-ray diffraction revealed that the particles were β-type siliconcarbide by chemical analysis, only silicon carbide and boron weredetected and the boron content was 0.8% by weight of the particles.According to X ray photoelectron spectroscopy the boron was elementalboron. Free carbon was not detected by the infrared absorptiometricmethod after combustion or hydrogen hot extraction technique, and theabsence of free carbon was thus confirmed.

The resultant silicon carbide particles were then subject to sintering.20 g of silicon carbide particles produced as above were transferredinto a glovebox filled with argon gas and containing 0.01 ppm or less ofoxygen, while the particles were prevented from coming into contact withair. In the glovebox, the particles were uniaxially formed at 90 kgf/cm²to make pellets and the pellets were charged in a rubber bag. The rubberbag with the pellets was discharged from the glovebox and the pellets ascharged in the rubber bag were statically pressed under a pressure of7000 kgf/cm² to form a shape. The shape was fired in a 1 atm argonatmosphere at 2080° C. for 2 hours. The temperature raising time fromroom temperature to 2080° C. was about 2 hours and 30 minutes. Thesinter was allowed to cool to room temperature after firing at 2080° C.

The resultant silicon carbide sinter had a density of 3.18 g/cm³ (by theArchimedean method), which corresponds to 99.1% of the theoreticaldensity.

EXAMPLE 6

Example 5 was repeated except that shaping was conducted in air withoutusing a glovebox.

The resultant silicon carbide sinter had a density of 3.16 g/cm³ (by theArchimedean method), which corresponds to 98.4% of the theoreticaldensity.

EXAMPLE 7

A silicon carbide sinter was produced by the same procedures as inExample 5 except that oleic acid was used as a binder for the shaping.Before uniaxial forming, the silicon carbide particles were added andmixed with 1 g of oleic acid solved in toluene and the mixture was driedto evaporate the acetone only. The oleic acid was evaporated afteruniaxial forming and static pressing was conducted. During the aboveprocedure, a glovebox was used and the silicon carbide was preventedfrom coming into contact with air.

The thus-obtained silicon carbide sinter had a density of 3.18 g/cm³ (bythe Archimedean method), which corresponds to 99.1% of the theoreticaldensity.

EXAMPLE 8

Example 7 was repeated except that the shaping was conducted in airwithout using a glovebox.

The resultant silicon carbide sinter had a density of 3.16 g/cm³ (by theArchimedean method), which corresponds to 98.4% of the theoreticaldensity.

We claim:
 1. A process for producing a boron-containing silicon carbidepowder, in which particulate silicon or gaseous silicon compoundcontaining no carbon, and particulate boron or a gaseous boron compoundcontaining no carbon, said particulate boron or said gaseous boroncompound being present in an amount of 0.1 to 5% by weight calculated asboron in the boron-containing silicon carbide to be produced, areintroduced into a first reaction zone in the absence of carbon at atemperature higher than the melting point of silicon to form a fusedboron-containg silicon particle; andsaid fused boron-containing siliconparticle is reacted with a carbon compound in a second reaction zone ata temperature lower than the boiling point of silicon to form theboron-containing silicon carbide powder.
 2. A process according to claim1, wherein said particulate silicon or silicon compound and saidparticulate boron or boron compound are preliminarily thoroughly mixedin a preselected ratio therebetween and the resultant thorough mixtureis introduced into said first reaction zone.
 3. A process according toclaim 1, wherein said boron compound is diborane.
 4. A process accordingto claim 1, wherein said carbon atom is an aliphatic hydrocarboncontaining 4 or less carbon atoms.
 5. A process according to claim 1,wherein said silicon compound is a member selected from the groupconsisting of silane, chlorosilane, disilane, silane tetrachloride andchloroalkylsilane.
 6. A process according to claim 1, wherein saidboron-containing silicn carbide powder comprises particles containingsubstantially no free carbon.